We performed electronic structure calculations within density functional theory including ab initio molecular dynamic simulations of isolated Fe13 and Fe55 clusters. We observed the energy preference of icosahedral clusters over cuboctahedrons for both Fe13 and Fe55. The magnetic structure is ferromagnetic for Fe13, but anti-ferromagnetic for Fe55. But isolated clusters exhibit a HOMO-LUMO gap.Subsequently, we embedded the clusters into models of silica glass of different size. For embedded clusters we observed the formation of an iron oxide interface between cluster and dielectric matrix. Though many different models were computed, they show a very homogeneous trend. The magnetization of optimized embedded models is close or larger than the magnetization of free clusters. The small clusters retain their electronic structure around the Fermi-level despite some major distortions.

Experimental island count histograms as a function of SAQD volume have been evaluated using an established model. The experimental data was obtained for 51 mm wafers grown by MOCVD and analyzed over the center 26 × 26 mm square of the wafer with AFM. More than one distribution is required for all conditions investigated to obtain adequate representations of the experimental data. Consistent parameters are obtained for samples grown with a variable InAs thickness. Higher growth temperatures results in material being converted into relaxed islands. Extended annealing without AsH3 eliminates small islands, suggesting that they are not a stable distribution.

The size selection of nanodots during the growth is modelled by using the reaction kinetic model with reaction rates for dot size dependent attachment and detachment processes, related to the free energy of dots. Long-lived metastable state is found near the minimum of free energy, but the kinetics of the growth causes the peak to overshoot the location of minimum of the energy. The size distribution of dots is shown to be nearly Gaussian with minor skewness originated from kinetics. However, it is argued that this skewness is inherently related to the effect of overshooting, and thus to the formation of the long lived metastable state with size selected nanodots. Based on RKM, a simple continuum model is derived to describe the size selection and narrowing of the size distribution.

We present in this work a method to control lateral ordering of In0.4Ga0.6As quantum dots (QDs) using the surface anisotropic and the growth environment. We have shown experimentally that using As2 molecules instead of As4 as a background flux is promising to control the diffusion of adatoms in away to make it possible to control the ordering of In0.4Ga0.6As QDs together with the GaAs surface properties specially for GaAs (100) surfaces. For GaAs (n11)B surfaces the As4 provides a better two dimension ordering than the As2 environment. Our results are consistent with reported experimental and theoretical studies on surface structure and diffusion mechanism over GaAs surface.

We present first results combining models at continuum and atomistic (DFT) levels to improve understanding of key mechanisms involved in silicon nanodots (NDs) synthesis on SiO2 by Low Pressure Chemical Vapor Deposition (LPCVD) from silane SiH4. In particular, by simulating an industrial LPCVD reactor using the CFD code Fluent, we find that the deposition time could be increased and then the reproducibility and uniformity of NDs deposition could be improved when highly diluting silane in a carrier gas. A consequence of this high dilution seems to be that the contribution to deposition of unsaturated species such as silylene SiH2 highly increases. This result is important since our first DFT calculations have shown that silicon chemisorption on silanol Si-OH or siloxane Si-O-Si bonds present on SiO2 substrates could only proceed from silylene (and probably from other unsaturated species). The silane saturated molecule could only contribute to NDs growth, i.e. silicon chemisorption on already deposited silicon bonds. Increasing silylene contribution to deposition in highly diluting silane could then also exalt silicon nucleation on SiO2 substrates and then increase NDs density.

The Hartree-Fock (HF), restricted open shell HF (ROHF), configuration interaction (CI), complete active space (ICASCF), and multiconfiguration self-consistent field (MCSCF) methods provide sophisticated fundamental theory-based, computational tools to study structure, composition,chemistry and electronic properties of small artificial molecules composed of semiconductor compound atoms. These tools are used to synthesize virtually several prismatic In-N and Zn-O artificial molecules whose structure is derived from that of the symmetry elements of the respective wurtzite bulk lattices. Applications of spatial constraints to the atomic coordinates allow modeling molecular synthesis in quantum confinement, to obtain pre-designed molecules with tunable electronic properties. Relaxation of these constraints, or optimization, leads to the corresponding molecules synthesized in “vacuum”. The development of computational templates of the studied artificial molecules synthesized in confinement reflects effects of quantum confinement on the electronic level structure, bonding, the direct optical transition energy, and charge and spin density distributions of the molecules. Comparison of the structure and properties of these molecules to those of their vacuum counterparts leads to a conclusion that a small changes in atomic positions in otherwise structurally similar molecules cause a significant change in their electronic properties. Thus, the electronic properties of artificial molecules can be tuned by changing their synthesis conditions that are defined by atomistic details of quantum confinement where the molecules are synthesized.

In this paper, a novel quantum dot (QD) based nanomaterial system is presented for efficient FRET analysis. The quantum dots have been embedded in hydrogel microspheres based on poly(N-isopropylacrylamide) (PNIPAM) a thermoresponsive polymer that undergoes a volume phase transition across the low critical solution (LCST). The optical properties of the quantum dots entrapped within the gel microspheres has been modified due to change in refractive index, volume density of the surrounding hydrogel medium. The QDs encapsulated in the PNIPAM microspheres showed 100–200 % enhancement in the PL efficiency as the microgels shrank at the temperature above the LCST temperature of the gel.

We have studied the temperature dependence of CER spectra of layered InGaAs QWRs and QDCs and found strain-induced splitting of lh and hh states occur in both InGsAs and GaAs layers. By fitting experimental data using Varshni law and Bose-Einstein type relation, various parameters are obtained, which are similar to those of bulk GaAs. We pointed out that a caution must be excised when extracting the electron-phonon interaction parameters by subtracting the thermal dilation part from the experimental data of the embedded semiconductor microstructures because in these structures the temperature-induced lattice-dilation may produce additional strain besides the lattice mismatch.

We report the fabrication of a one dimensional microcavity structure embedded with colloidal CdSe/ZnS core/shell quantum dots using solution processing. The microcavity structures were fabricated by spin coating alternating layers of polymers of different refractive indices (poly-vinylcarbazole, and poly-acrylic acid) to form the distributed Bragg reflectors (DBRs). Greater than 90% reflectivity was obtained using ten periods of the structure. The one dimensional microcavity was formed by sandwiching a λ/n thick defect layer between two such DBRs. The microcavity demonstrated directionality in emission and well behaved dispersion characteristics. Room temperature time-resolved photoluminescence measurements carried out on this structure showed six fold enhancement of spontaneous emission rate. The photoluminescence decay time of the quantum dots was found to be ∼ 1 ns while for the quantum dots embedded in the microcavity it was ∼150 ps.

We report the growth and structural properties of InSb and InSb:N quantum dots on InAs and GaAs substrates. Strain induced, self-assembled quantum dots are grown using solid-source molecular beam epitaxy. For improved growth control, we developed a growth technique similar to atomic layer epitaxial methods. InSb and InSb:N multiple quantum dots formed on both InAs and GaAs. We explain the formation of multiple quantum dots by the anisotropic distribution of strain energy within the quantum dot, the long adatom lifetime during atomic layer epitaxy, and the low bond energy of InSb. Nitrogen incorporation during formation of quantum dots changes surface energy barrier and causes anisotropic distribution of strain energy, results in formation of closely coupled multiple quantum dots in <110> orientation. We obtained mid infrared luminescence around 3.6 μm from InNSb QDs grown on InAs substrate, where it exhibits relatively low efficiencies of nitrogen incorporation compared to the quantum well structure.

Self-assembled InAs and InP quantum dot molecules (QDMs) are grown on GaAs substrates using different molecular beam epitaxial (MBE) growth techniques. The structural and optical properties of the two types of QDMs are then compared and reported. Multi-stack high-density (1012 cm-2) InAs QDMs are grown and when inserted into GaAlAs/GaAs heterostructure results in high-efficiency solar cells. As an alternative to InAs, InP QDMs are grown by droplet epitaxy of In and annealing under P2 pressure. While the number of quantum dots per QDM in the case of InP is in the range of 10 to 12 dots, those in the case of InAs can be smaller or much larger depending on exact growth parameters prior to QD growth. Photoluminescence (PL) measurements show that while InAs QDMs provide room-temperature optical output that peaks at 1.1 eV, InP QDMs have no PL output, possibly due to crystal defects created by low-temperature processing associated with droplet epitaxy. Discussion on the practicality of our QDMs as material for intermediate band solar cells is also provided.

Lateral ordering of InGaAs quantum dots over GaAs (001) has been achieved in earlier reports resembling anisotropic pattern. We present in this letter a method of breaking the anisotropy of ordered QDs by changing the growth environment. We do show experimentally that using As2 molecules instead of As4 as a background flux is effective in controlling the diffusion of Ga adatoms in away to make it possible to produce isotropic ordering of InGaAs QDs over GaAs (001). Our results are consistent with reported experimental and theoretical studies on surface structure and diffusion mechanism over GaAs.

The purpose of this study is the assessment of the properties of deposited atomic chains. Therefore linear chains of covalent and metallic atoms, i.e. As and Ag,deposited onto monolayer SA steps onto Si(100) have been considered. Their study is based on a semi-empirical Hamiltonian, used for the evaluation of both the electronic structure and the conductance, and the calaculations analyze the binding energy of chains of variable length deposited onto steps in the light of the analogous energy of free-standing chains and of chains deposited onto the Si(100) surface. This comparison shows that the stability of the chains has a primary dependence on the type of the substrate, rather than on the chain length and composition, and increases in the order: free standing,deposited onto SA,deposited onto Si(100). The central result of the calculations of the conductance is that the dependence on the type of the substrate and on the chain composition parallels the one of the binding energy.

We have proposed a novel self-assembling growth method, termed Droplet Epitaxy, for the direct formation of QDs without using any lithography in 1990. Compared with the island formation based on the Stranski-Krastanow growth mode, the Droplet Epitaxy is applicable to the formation of quantum dots not only in lattice-mismatched but also in lattice-matched systems such as GaAs/AlGaAs. The process of the Droplet Epitaxy in MBE chamber consists of forming numerous III-column element droplets such as Ga or InGa with homogeneous size of around 10 nm on the substrate surface first by supplying their molecular beams, and then reacting the droplets with As molecular beam to produce GaAs or InGaAs epitaxial microcrystals.Another advantage of the Droplet Epitaxy is the possibility of the fabrication of QDs structures without wetting layer by cotrolling the stoichiometry of the substrate surface just before the deposition of III-column element droplets. Also we can control the shape of the QDs structure self-organizingly such as pyramidal shape, single-ring shape and concentric double-ring shape. These ring structures will provide excellent possibilities for the investigation of quantum topological phenomena.

The size of germanium (Ge) nanocrystals in a trilayer memory device structure was controlled by varying the thickness of the middle co-sputtered Ge plus silicon oxide layer. Such confinement of nanocrystals was not effective in a trilayer structure with a pure Ge middle layer. Significant diffusion of Ge atoms through the tunnel oxide or rapid thermal oxide (RTO) layer and into the silicon substrate was observed when the RTO layer thickness of the trilayer structure was reduced. This resulted in no (or very few) nanocrystals formed in the system. A higher charge storage capability was obtained from devices with a thinner RTO layer.